Water weight makes up the majority of the total net weight of potable water systems on-board aircraft and other passenger transport vehicles. During aircraft takeoff, this weight becomes particularly expensive in terms of fuel requirements. There exists a large incentive to reduce this weight. However, potable water is required for lavatory and galley uses. In the lavatory, potable water is required to wash users' hands, but potable water is also used for the toilet flushing. It is desirable to provide a way to re-use the spent water from the hand-washing process on-board. During the hand washing process, after the potable water exits the faucet for rinsing the user's hands, the spent water (also referred to as “grey water”) is considered waste and is typically discharged overboard or introduced to the waste system.
While the water being dispensed from the faucet is scrutinized by regulatory agencies for quality and health standards, the water used to flush the toilet is not under the same restrictions. There are no regulations preventing the use of this sink discharge, the “grey water,” for flushing the toilet. There may be other on-board uses for the grey water as well. Ground-based uses of collected grey water include flushing toilets, watering gardens, and other non-potable consumptions of water. However, these collection and re-use systems are ill-suited for aircraft installation due to health, structural, and environmental requirements.